1. Technical Field
The present invention relates to electromagnetic wave propagation, and more particularly to devices and methods for generating terahertz electromagnetic waves using graphene.
2. Description of the Related Art
Terahertz emission refers to electromagnetic waves with frequencies ranging roughly from 0.1 to 10 THz, which is between the conventional microwave and optical frequencies. Generation of terahertz electromagnetic waves is of great scientific and technological importance. Terahertz electromagnetic (EM) waves have numerous applications in medical imaging, security, communications, manufacturing, and scientific spectroscopy. Coherent photon-mixing is one of the most promising ways to generate Terahertz EM waves. Compared with other approaches such as quantum cascade lasers and synchrotron light sources, its advantages include compact device size, room temperature operation, and continuous tunability.
To efficiently generate terahertz emission in a wide frequency range, the following three important requirements should be met. First, the carrier lifetime should be short (ideally in sub-ps range) to generate emission with a frequency larger than a few THz. Second, the carrier mobility should be as high as possible to maximize the photoconductivity of the material. Third, the heat generation should be minimized to achieve high terahertz output power.
Currently, the most commonly used material in coherent photon-mixing is low-temperature grown Gallium Arsenide (LT-GaAs). The major motivations for using this material are its short photo-carrier lifetime and relatively good carrier mobility (e.g., 100 to 200 cm2/Vs).
There are two intrinsic limitations in these conventional photonic materials. These include: 1) short photo-carrier lifetime and high mobility cannot be realized simultaneously in conventional photonic semiconductors; and 2) heat generation is significant since the energy of photons used for THz generation has to exceed the band gap of the semiconductors. Conventional high-quality III-V materials usually have high carrier mobility (especially high electron mobility). At the same time, the photo-carrier lifetime is long (in the nano-second range). To reduce the photo-carrier lifetime for high frequency THz generation, the quality of material is usually intentionally degraded by introduction of defects or dopants. These inevitably lead to reduction in carrier mobility and hence a decrease in photo-conductivity. A compromise is achieved in LT-GaAs, in which, the carrier lifetime is ˜1 ps and the carrier mobility is around 100 cm2/Vs. A large portion of the energy of the incident photons in THz generation using photon-mixing is dissipated by heat.